A magnetic disk drive typically includes a stack of spaced apart, concentric magnetic disks mounted on a common spindle. Disposed adjacent to the stack of disks is an actuator arm assembly which comprises a plurality of arms extending into the spacings between the disks. Mounted on the distal end of each arm is a resilient load beam which in turn carries a magnetic head interacting with an associated magnetic disk.
FIG. 1 shows schematically a top plan view of a typical prior art magnetic disk drive signified by reference numeral 2. The disk drive 2 includes an arm 4 revolvable about an arm axis 6, and a magnetic disk 8 rotatable about a spindle 10. There is a multiplicity of concentric data tracks 12 registered on the surface of the disk 8. Attached to the distal end of the arm 4 is an air bearing slider 13 carrying a magnetic head 14 which interacts with the magnetic disk 8. During normal operation, the disk 8 spins at high speed in a rotational direction 15. The aerodynamics of the moving air between the slider 13 and the surface of the disk 8 provides sufficient buoyancy to suspend the slider 13 above the disk surface. The height of the slider 13 above the disk surface is called the flying height of the magnetic head 14. To gain access to each data track 12, the arm 4 sweeps the head 14 in an arcuate locus 16 traversing the disk surface.
Attention is now directed to the angle formed by the center line through the slider 13 and the tangent line to each data track 12. This angle is defined as the skew angle of a particular data track. The tangent line to the outermost track OD is labeled T.sub.OD, and the center line passing through the slider 13 is signified by reference numeral 18. The angle between the center line 18 and the tangent line T.sub.OD of the outermost track OD is defined as the skew angle .theta..sub.OD. Likewise, the angle between the center line 18 and the tangent line T.sub.ID of the innermost track ID is defined as the skew angle .theta..sub.ID. It should be noted that the skew angles .theta..sub.OD and .theta..sub.ID may be different in magnitude and polarity, depending upon factors such as the arm length and relative position of the arm 4 with respect to the disk 8. With the center line 18 as reference, the skew angle .theta..sub.ID is negative (measured counterclockwise) while the skew angle .theta..sub.OD is positive (measured clockwise). With the arm axis 6 relatively far away from the spindle 10, the angle .theta..sub.OD is also smaller that the angle .theta..sub.ID in absolute value. For data tracks in between, the polarities and angular magnitudes range between these extremes.
The skew angle is of significant importance in the design of a magnetic disk drive. To begin with, the skew angle dictates the angular orientation of the slider 13 with respect to the air stream while the disk 8 is spinning in the direction 15. Accordingly, the dynamic air pressures exerted on the slider 13 are different at different data tracks. As a result, the slider 13 carrying the magnetic head 14 flies at different heights above the disk surface at different skew angles. However, signal intensity sensed by the head 14 during the read mode, or written onto the disk 8 during the write mode, is a strong function of the flying height. Therefore, performance of the magnetic head 14 varies at different skew angles.
To compound the situation further, the radial distance of the magnetic head 14 away from the spindle center 20 also plays an important role in the determination of the flying height. While the disk 8 is spinning, the outermost track OD experiences higher linear velocity relative to the innermost track ID. Dynamic pressure exerted on the slider 13, also a function of linear velocity, is correspondingly higher at the outermost track OD compared to the innermost track ID. In conjunction with the skew angle factors, the flying height of the magnetic head 14 is indeed difficult to predict theoretically.
For the above reasons, in the design and manufacturing of magnetic disk drives, there is a need for a tester which is capable of duplicating the skew angle and the radial distance of the magnetic head 14 in each of the data tracks 12 accurately.
Various disk drive testers have been proposed in the past. In U.S. Pat. No. 4,902,971, entitled "Magnetic Head and Disc Tester Employing Pivot Arm on Linearly Movable Slide", issued Feb. 20, 1990, a magnetic disk and head tester is disclosed. The tester includes an arm having a proximal end fixedly secured onto a translational slide, and a distal end attached with a magnetic head. To operate the tester, the skew angle of the outermost and innermost tracks need first be determined. Thereafter, the distance between the slide and the disk spindle, and the angular orientation of the arm are all correspondingly adjusted, such that the arm forms the predetermined skew angles at the outermost and innermost tracks. The arm is then fixedly tightened onto the slide. Linear motion of the slide carrying the arm traversing the disk is utilized to simulate the rotational motion of the arm of a disk drive. Errors can be induced for data tracks located between the outermost and innermost tracks. The situation would be more aggravated with smaller disk drives having shorter arms which sweep arcs of smaller radii of curvature.
To rectify this problem, another tester has been proposed. In U.S. Pat. No. 5,254,946, entitled "Magnetic Head and Disk Tester with Head Centrally Mounted on Radially-Moving, Rotatable Platform that Surrounds Disk", issued Oct. 19, 1993, the tester includes a testing arm fixedly attached to a rotatable housing which is also capable of linear movement. The disk under test is positioned underneath the housing and the arm. Prior to usage, with the aid of a charged coupled device (CCD) camera, the magnetic head must first be manually and accurately aligned with the center of rotation of the housing. Pretested preparation of this type of setup is time consuming. Moreover, for the testing of different disks, considerable steps of mounting and demounting are involved. Testers of this type are not suitable for mass production environments.
Still, there are other testers employing X-Y manipulators to gain access to the different data tracks on magnetic disks. Testers of this type involve very complicated head traveling patterns and are not easy to operate.
None of the prior art testers provide any features that are suitable for high throughput operation. There is a need for magnetic head and disk testers that can fulfill fast development turnaround and high volume production requirements.